Anadromy and residency in steelhead and rainbow trout (Oncorhynchus mykiss): a review of the processes and patterns PublicDeposited

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Descriptions

Oncorhynchus mykiss form partially migratory populations with anadromous fish that undergo marine migrations and
residents that complete their life cycle in fresh water. Many populations’ anadromous components are threatened or endangered,
prompting interest in understanding ecological and evolutionary processes underlying anadromy and residency. In this
paper, we synthesize information to better understand genetic and environmental influences on O. mykiss life histories, identify
critical knowledge gaps, and suggest next steps. Anadromy and residency appear to reflect interactions among genetics,
individual condition, and environmental influences. First, an increasing body of literature suggests that anadromous and
resident individuals differ in the expression of genes related to growth, smoltification, and metabolism. Second, the literature
supports the conditional strategy theory, where individuals adopt a life history pattern based on their conditional status relative
to genetic thresholds along with ultimate effects of size and age at maturation and iteroparity. However, except for a generally
positive association between residency and high lipid content plus a large attainable size in fresh water, the effects of body size
and growth are inconsistent. Thus, individuals can exhibit plasticity in variable environments. Finally, patterns in anadromy and
residency among and within populations suggested a wide range of possible environmental influences at different life stages,
from freshwater temperature to marine survival. Although we document a number of interesting correlations, direct tests of
mechanisms are scarce and little data exist on the extent of residency and anadromy. Consequently, we identified as many data
gaps as conclusions, leaving ample room for future research.